Night-time control of carbohydrate availability in grasses differs radically from that in Arabidopsis

Grass leaves reportedly accumulate sucrose during the day then export it to other organs at night. The control of export at night is not understood. In contrast, Arabidopsis leaves accumulate starch then convert it to sucrose for export at night. Control of starch mobilisation ensures a constant sucrose supply and exhaustion of starch around anticipated dawn. We found that that 41 species of pooid grasses differed widely in the ratio of sucrose to starch accumulation and night-time depletion (turnover) in leaf blades, regardless of growth conditions. Sucrose turnover exceeded starch turnover in most species, but in a minority of species the two were similar. In eight species spanning this diversity, the rate of sucrose depletion fell through the night. In six of the species, all with higher sucrose than starch turnover, starch depletion at night was initially slow. Later, it increased to a rate that exhausted starch around dawn. The initial lag was absent and starch depletion was linear throughout the night in the other two species, both of which had similar sucrose and starch turnover. We suggest that underlying control of starch mobilisation in grass leaf blades resembles that in Arabidopsis leaves. The initial lag in starch depletion in high-sucrose species may result from suppression of starch degradation by sucrose, acting via the sucrose signalling metabolite Tre6P. The large diel fluctuations in sucrose export from grass leaf blades may be dampened by exchange with dynamic pools of sucrose in leaf bases and sheaths prior to export to other organs.